1. Field of the Invention
The invention relates to a projection system, and especially relates to a touch projection system.
2. Description of the Prior Art
Recently, touch applications develop rapidly. Some kinds of projectors are designed to cooperate with touch technology. Common touch technology at present is to form an infrared light curtain in front of a screen. A receiving module is added into a projector for receiving image light produced by the infrared light curtain, so that a touch operation of a user can be determined by analyzing an image of the user interrupting the infrared light curtain. In practice, the receiving module can be integrated with a projection lens of the projector. The image light enters the projector through the projection lens and is reflected by a micromirror device (e.g. a digital micromirror device, DMD) to an image-capturing device. In this case, the micromirror device performs the function of modulating (e.g. selectively reflecting) light produced by a light source device of the projector and projecting the modulated light onto the screen to form images, and also the function of reflecting the image light, which enters the projector, to be received by the image-capturing device. The micromirror of a common digital micromirror device usually offers two states. One is defined as ON state, at which the micromirror reflects light to be projected toward the screen by the projection lens. The other is defined as OFF state, at which the micromirror reflects light to deflect off the projection lens so that the light is prevented from being projected onto the screen. In general, the micromirror rotates relative to a single axis to offers the ON state and the OFF state based on the limit positions of positive rotation and negative rotation. Thereby, the components of the projector can be disposed with less structural interference and operate normally. Therefore, the micromirror device uses the OFF state to reflect the image light to the image-capturing device. However, the rotation angle of the micromirror is restricted so that it is hard to configure the components of the projector compactly leading to incapability of effectively reducing the volume of the projector.